Environmental fate and effects of dicamba: a Canadian perspective

Literature on the environmental fate and effects of the benzoic acid herbicide dicamba was reviewed to provide a scientific basis to derive Canadian Water Quality Guidelines. Included in the review was information on the uses and production of dicamba, its physical and chemical properties, environmental monitoring data in Canadian surface water and groundwater, soils, sediments, and biota, and its environmental degradation, persistence, and fate. Through monitoring, dicamba has been detected in less than 8% of surface-water samples to a maximum concentration of 13 micrograms.L-1, while 2% of groundwater samples were positive up to 517 micrograms.L-1. Only one study that analyzed sediments (with no detections) and no field studies that investigated residues in biota were found. Microbial degradation is the most important process governing the dissipation of dicamba in aquatic and soil environments. Photolysis, hydrolysis, volatilization, adsorption to sediment, and bioconcentration are not expected to be significant removal processes, based on limited environmental fate data. The half-life of dicamba in water is < 7 d, although residues have been detected in surface-water supplies in Alberta more than 6 mon after application. The literature reports the half-life in soils ranges from 4 to 555 d; however, < 12 wk would be typical under Canadian conditions. High moisture and temperature, and other conditions that favor microbial degradation, would likely reduce the half-life to < 4 wk. The principal soil and plant metabolite is 3,6-dichlorosalicylic acid, with minor amounts of 2,5-dihydroxy-3,6-dichlorobenzoic acid and 5-hydroxydicamba found. Dicamba is highly mobile in soil, and significant leaching is possible; its water solubility is 6.5 g.L-1 (25 degrees C) and it has a log octanol-water partition coefficient of 0.477. Acute and chronic toxicological studies for all nontarget plants and animals were also reviewed. The major groups of organisms for which toxicological data were collected were freshwater fish, invertebrates and plants, tame hays and cereals, legumes, and other crops, and livestock poultry and mammals. The acute toxicity (< or = 96-hr LC50) to freshwater fish ranged from 28 to 516 mg.L-1, whereas that for invertebrates ranged from 3.9 to > 100 mg.L-1. No chronic data were found for either of these groups. The chronic EC50 to 14 freshwater algae, based on growth inhibition, ranged from 100 to > 10,000 micrograms.L-1. No studies on freshwater macrophytes or any marine organisms were found. Agricultural crops exhibited varying toxicity.(ABSTRACT TRUNCATED AT 400 WORDS)